Add BETA warning to Phabricator Importer doc

Problem to solve

The Phabricator Importer documentation page lists exactly what the importer does. However, it fails to convey the overall completeness of the importer, compared to other importers. This importer only does a basic import that creates an issues-only project and copies into it the issues from Phabricator containing only a few fields. This functionality may not be enough for many users and we need to make sure that this is explicitly reflected in the documentation.

Proposal

Add a BETA warning similar to the image below to the top of the Phabricator doc page:

This warning should explicitly state that this importer is not complete and only imports the issues with titles and descriptions. It should also call out the fact that GitLab project created will only contain issues and have the repository disabled.

Who can address the issue

Anyone familiar with editing GitLab documentation.